1. Field of the Invention
The present invention relates to a structure for supporting compressive loads with a high strength-to-weight efficiency and, more particularly, to a structure which can be used in the fuselage of a rocket or aircraft or to form fuel tanks.
2. Description of the Related Art
In applications where it is desirable for a structure to be as light as possible while withstanding relatively high loads, e.g., in an aircraft or rocket, many types of structures have been used. In the prior art, the highest efficiency structure for carrying a compressive load utilizes a corrugated panel construction. This structure is illustrated in FIGS. 1-3 and includes beaded-web corrugation 20 bonded to cap strips 22 at uncorrugated regions or "flats" 24 at bond lines 26 (FIG. 2). As illustrated in cross-section in FIG. 3, the corrugated portion of the beaded web 20 has an undulating or sinusoidal cross-section.
The beaded web 20 is typically superplastic formed of very thin sheets with concurrent diffusion bonding (SPF/DB) to the curved cap strips 22. As described in U.S. Pat. No. 4,292,375, the SPF/DB method of fabrication utilizes two inherent phenomena which tend to occur concurrently in, e.g., titanium alloys. The first phenomenon is the ability of a material, such as titanium alloy, to undergo large, up to 1,000 percent strain, plastic deformations at high temperatures without localized thinning, or necking. This phenomenon often is referred to as superplasticity. The second phenomenon relates to the capability of being joined under pressures at elevated temperatures, without melting or the use of bonding agents, which is referred to as diffusion bonding.
In an optimized design the corrugation structure illustrated in FIG. 1 may have an overall thickness of two to three (2-3) inches from the top of a curved cap 22 on one side to the top of another cap 22 curved in the opposite direction. The beaded-web 20 and cap strip 22 may have thicknesses of two to five (2-5) mil and thirty to sixty (30-60) mil, respectively. Since FIGS. 1-3 are approximately to scale for the above "optimized design", a good approximation for other dimensions can be obtained from FIGS. 1-3.
In this "optimized designs", the beaded-web material 20 accounts for two-thirds of the total structural mass. The web material 20 is beaded so that it provides a load carrying capacity in a direction perpendicular to the compressive load and the "flats" to which the cap strips 22 are bonded are so thin they provide an insignificant amount of load carrying capability. Consequently, 67% of a beaded-web, curved-cap corrugation structure, constructed as described above, carries very little of the compressive load. When the compressive loading is limiting, there is considerable inefficiency and limiting, there is considerable inefficiency and potential room for improvement.
In the beaded-web, curved-cap structure, the cap strips 22 carry the compressive load. The thickness-to-width ratio and the edge restraint of the cap strips 22 determine the ability of the cap strips 22 to resist local buckling. The beading in the web 20 helps prevent local buckling in the web 20 while allowing maximum separation between the strips 22. The separation between the cap strips determines the stiffness of the corrugation structure to resist overall buckling. The bead depth d (FIG. 1) helps provide edge restraint for the cap strips 22, but introduces load discontinuities due to the variation in the amount of the cap strip 22 which is supported by the beaded web 20.